1. Field of the Invention
The present invention relates to a method of storing hydrogen in a macroporous monolithic material, in particular in a carbon or ceramic monolith having an M2 (macroporous/microporous) hierarchized porous structure and also to a composite for storing hydrogen and to a gaseous hydrogen production process employing such a composite.
2. Description of Related Art
Materials in the form of porous carbon monoliths constitute materials of choice for many applications such as the purification of water and air, adsorption, catalysis in heterogeneous phase, manufacture of electrodes, and energy storage because of their high specific surface area, their large pore volume, their insensitivity to surrounding chemical reactions and their excellent mechanical properties.
These materials have a high specific surface area and a hierarchized structure, i.e. a cellular structure generally having two types of porosity. In particular, they have a mesoporous structure in which the mean pore diameter varies from around 2 to 10 nm.
They may be prepared according to two broad families of processes.
The first broad family of processes uses soft templates and corresponds to what are called “soft templating” methods, i.e. methods employing organic-organic interactions between a thermally crosslinkable polymer (generally a carbon precursor) and certain block copolymers of the nonionic polymer type, such as the products sold under the brand name Pluronic® P123 or F127 by the company BASF, which are used as modeling agent for directly obtaining a porous carbon material after carbonization in an inert atmosphere at 350° C. and pyrolysis (Y. Meng et al., Angew. Chem. Int. Ed., 2005, 44, 2).
The second broad family of processes uses rigid templates and corresponds to what are called “hard templating” or “exotemplating” methods, i.e. methods in which a mesoporous solid template is impregnated with a solution of a precursor of the final material that it is desired to obtain (for example, a carbon precursor) before being carbonized in a nonoxidizing atmosphere.
The invention described hereinbelow employs materials prepared according to a process belonging to the family of hard templating methods.
More precisely, hard templating methods employ templates that may in particular consist of mesoporous silica particles, alumina membranes, zeolites, etc. In particular it is known, for example, from the article by K. P. Gierszal et al., New Journal of Chemistry, 2008, 32, 981-993 that mesoporous carbon monoliths may be prepared by a method using mesoporous silica matrices of the MCM-48 and KIT-6 type having a cubic 3D structure. According to this method, the silica matrices, in powder form, are firstly impregnated with a solution of a carbon precursor which is then crosslinked within the matrices, and then the matrices are carbonized. The carbon material is finally obtained after elimination of the silica template by an acid treatment. It takes the form of a mesoporous material, the pores of which have a mean diameter between 3 and 5 nm approximately and the porous network of which has a structure that corresponds substantially to the negative of the porous network of the template used. Moreover, the preparation of carbon monoliths comprising a macroporous/mesoporous network by impregnation of a solid mesoporous/macroporous silica template with a solution of a carbon precursor such as furfuryl alcohol, which is then polymerized within the template before the latter is eliminated, for example by an acid treatment, has also been described in particular by S. Alvarez et al., Advanced Engineering Materials, 2004, 6(11), 897-899. The carbon monoliths obtained according to the method described by S. Alvarez et al. comprise a macroporous network, which is the positive replica of the macroporous network of the silica template used, and a mesoporous network that corresponds to the negative of the mesoporous network of the silica template used.
At the present time, there is no hard templating method for obtaining materials having a hierarchized porous network comprising a macroporous network, which is the exact replica of the macroporous network of the silica template used, and a microporous network, while still containing no mesoporous network.
Now, it is advantageous to be able to have materials of this type insofar as the presence of a macroporous network enables the impregnation of a fluid such as a reactant within the material to be improved. It is also beneficial to be able to have a material having a certain microporosity, as this increases the specific surface area of the material and improves the confinement reactions in the gas phase (for example, enabling hydrogen to be stored in the gas phase). However, the presence of a mesoporous network in a material that also has a macroporous network is not always desirable as this would have the consequence of embrittling the texture of the watts and therefore impairing the mechanical properties of the material in its entirety. Furthermore, it has been recently demonstrated that the presence of a mesoporous network is not necessary for obtaining high-performance heterogeneous catalysts (S. Ungureanu et al., Chem. Mater., 2008, 20, 6464-6500).
The storage and production of dihydrogen also represent a current major challenge because of technological evolution and the exhausting of petroleum resources. The obsession for portable technologies is generating an increasing demand for systems enabling dihydrogen to be stored and produced in a simple and industrializable manner.
During the last ten years, many research studies have been undertaken to develop technologies intended to allow dihydrogen to be stored under satisfactory conditions from the standpoint of both safety and use. Among such technologies, mention may in particular be made of materials for high-pressure storage reservoirs, dihydrogen liquefaction processes and dihydrogen and metal hydride adsorption materials, these being adapted to a wide variety of both stationary and portable applications. The use of metal hydrides has the advantage of allowing pure hydrogen to be produced. Metal hydrides, especially borohydrides, also have a very high capacity (mass capacity and especially volume capacity) in terms of hydrogen storage. The production of dihydrogen using borohydrides may take place by hydrolysis according to the following theoretical reaction:M(BH4)x+2H2O→M(BO2)x+4H2 (with M=Li, Na, Mg, etc.).
In this case, the dihydrogen may be produced on demand, but no treatment for recycling the borates M(BO2)x into borohydrides is presently viable from an economic standpoint. The other means for desorbing large quantities of hydrogen consists in thermally desorbing the borohydrides by heating them to a high temperature, generally to temperatures equal to or greater than 400° C.